Bibliography - Richard T Wetherald

Wetherald, Richard T., December 2011: The role of low clouds in determining climate sensitivity in response to a doubling of CO₂ as obtained from 16 mixed-layer models. Climatic Change, 109(3-4), DOI:10.1007/s10584-011-0047-3.
Abstract
The effects that low clouds in sub-tropical to tropical latitudes have in determining a given model’s climate sensitivity is investigated by analyzing the cloud data produced by 16 “slab” or mixed-layer models submitted to the PCMDI and CFMIP archives and their respective response to a doubling of CO2. It is found that, within the context of the 16 models analyzed, changes of these low clouds appear to play a major role in determining model sensitivity but with changes of middle cloud also contributing especially from middle to higher latitudes. It is noted that the models with the smallest overall cloud change produce the smallest climate sensitivities and vice versa although the overall signs of the respective cloud feedbacks are positive. It is also found that the amounts of low cloud as simulated by the respective control runs have very little correlation with their respective climate sensitivities. In general, the overall latitude-height patterns of cloud change as derived from these more recent experiments agree quite well with those obtained from much earlier studies which include increases of the highest cloud, decreases of cloud lower down in the middle and lower tropospheric and small increases of low clouds. Finally, other mitigating factors are mentioned which could also affect the spread of the resulting climate sensitivities.
Wetherald, Richard T., October 2010: Changes of time mean state and variability of hydrology in response to a doubling and quadrupling of CO₂. Climatic Change, 102(3-4), DOI:10.1007/s10584-009-9701-4.
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This paper examines the subject of hydrologic variability and its changes in two separate integrations of a coupled ocean-atmosphere general circulation model developed at the Geophysical Fluid Dynamics Laboratory/NOAA assuming a 1% per year increase to a doubling and quadrupling of CO2, respectively. Changes in time mean state and variability of precipitation, runoff and soil moisture are evaluated using monthly and seasonal mean data derived from these integrations. Various statistical tests are then performed on the resulting time mean and variability changes. The patterns of hydrologic change for these three quantities are similar to those obtained from previous studies. In northern middle to higher latitudes for the time means, the changes include increases in monthly mean precipitation, increases in monthly mean runoff during the fall, winter and spring seasons and decreases of monthly mean soil moisture during summer. Many of these changes are found to be statistically significant at the 5% significance level for both the time mean and variability especially for the results where CO2 is quadrupled such as monthly mean precipitation. Significant changes also include increases of runoff variability during spring, winter and spring and increases of soil moisture variability during the summer season. These results support statements made in previous IPCC reports that increasing greenhouse gases can lead to more severe and frequent floods and droughts depending upon season and latitude. This study also indicates that the approaches to equilibrium of these two integrations, and the resulting hydrologic changes, take place over time scales of hundreds of years in agreement with several previous investigations.
Wetherald, Richard T., November 2009: Changes of variability in response to increasing greenhouse gases Part II: Hydrology. Journal of Climate, 22(22), DOI:10.1175/2009JCLI2834.1.
Abstract
This paper examines hydrological variability and its changes in two different versions of a coupled ocean–atmosphere general circulation model developed at the National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory and forced with estimates of future increases of greenhouse gas and aerosol concentrations. This paper is the second part, documenting potential changes in variability as greenhouse gases increase. The variance changes are examined using an ensemble of 8 transient integrations for an older model version and 10 transient integrations for a newer model. Monthly and annual data are used to compute the mean and variance changes. Emphasis is placed on computing and analyzing the variance changes for the middle of the twenty-first century and compared with those found in the respective control integrations. The hydrologic cycle intensifies because of the increase of greenhouse gases. In general, precipitation variance increases in most places. This is the case virtually everywhere the mean precipitation rate increases and many places where the precipitation decreases. The precipitation rate variance decreases in the subtropics, where the mean precipitation rate also decreases. The increased precipitation rate and variance, in middle to higher latitudes during late fall, winter, and early spring leads to increased runoff and its variance during that period. On the other hand, the variance changes of soil moisture are more complicated, because soil moisture has both a lower and upper bound that tends to reduce its fluctuations. This is particularly true in middle to higher latitudes during winter and spring, when the soil moisture is close to its saturation value at many locations. Therefore, changes in its variance are limited. Soil moisture variance change is positive during the summer, when the mean soil moisture decreases and is close to the middle of its allowable range. In middle to high northern latitudes, an increase in runoff and its variance during late winter and spring plus the decrease in soil moisture and its variance during summer lend support to the hypothesis stated in other publications that a warmer climate can cause an increasing frequency of both excessive discharge and drier events, depending on season and latitude.
Stouffer, Ronald J., and Richard T Wetherald, 2007: Changes of Variability in Response to Increasing Greenhouse Gases. Part I: Temperature. Journal of Climate, 20(21), 5455-5467.
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This study documents the temperature variance change in two different versions of a coupled ocean–atmosphere general circulation model forced with estimates of future increases of greenhouse gas (GHG) and aerosol concentrations. The variance changes are examined using an ensemble of 8 transient integrations for the older model version and 10 transient integrations for the newer one. Monthly and annual data are used to compute the mean and variance changes. Emphasis is placed upon computing and analyzing the variance changes for the middle of the twenty-first century and compared with those found in a control integration. The large-scale variance of lower-tropospheric temperature (including surface air temperature) generally decreases in high latitudes particularly during fall due to a delayed onset of sea ice as the climate warms. Sea ice acts to insolate the atmosphere from the much larger heat capacity of the ocean. Therefore, the near-surface temperature variance tends to be larger over the sea ice–covered regions, than the nearby ice-free regions. The near-surface temperature variance also decreases during the winter and spring due to a general reduction in the extent of sea ice during winter and spring. Changes in storminess were also examined and were found to have relatively little effect upon the reduction of temperature variance. Generally small changes of surface air temperature variance occurred in low and midlatitudes over both land and oceanic areas year-round. An exception to this was a general reduction of variance in the equatorial Pacific Ocean for the newer model. Small increases in the surface air temperature variance occur in mid- to high latitudes during the summer months, suggesting the possibility of more frequent and longer-lasting heat waves in response to increasing GHGs.
Randall, David A., Michael E Schlesinger, V Y Galin, V Meleshko, J-J Morcrette, and Richard T Wetherald, 2006: Cloud feedbacks In Frontiers of Climate Modeling, Kiehl, J T, V Ramanathan, eds., UK, Cambridge University Press, 217-250.
Manabe, Syukuro, P C D Milly, and Richard T Wetherald, August 2004: Simulated long-term changes in river discharge and soil moisture due to global warming. Hydrological Sciences, 49(4), 625-642.
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By use of a coupled ocean-atmosphere-land model, this study explores the changes of water availability, as measured by river discharge and soil moisture, that could occur by the middle of the 21st century in response to combined increases of greenhouse gases and sulphate aerosols based upon the "IS92a" scenario. In addition, it presents the simulated change in water availability that might be realized in a few centuries in response to a quadrupling of CO2 concentration in the atmosphere. Averaging the results over extended periods, the radiatively forced changes, which are very similar between the two sets of experiments, were successfully extracted. The analysis indicates that the discharges from Arctic rivers such as the Mackenzie and Ob' increase by up to 20% (of the pre-Industrial Period level) by the middle of the 21st century and by up to 40% or more in a few centuries. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra rivers increase substantially. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller, with both positive and negative signs. For soil moisture, the results of this study indicate reductions during much of the year in many semiarid regions of the world, such as the southwestern region of North America., the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. As a percentage, the reduction is particularly large during the dry season. From middle to high latitudes of the Northern Hemisphere, soil moisture decreases in summer but increases in winter.
Manabe, Syukuro, Richard T Wetherald, P C D Milly, Thomas L Delworth, and Ronald J Stouffer, May 2004: Century-scale change in water availability: CO₂-quadrupling experiment. Climatic Change, 64(1-2), DOI:10.1023/B:CLIM.0000024674.37725.ca.
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It has been suggested that, unless a major effort is made, the atmospheric concentration of carbon dioxide may rise above four times the pre-industrial level in a few centuries. Here we use a coupled atmosphere-ocean-land model to explore the response of the global water cycle to such a large increase in carbon dioxide, focusing on river discharge and soil moisture. Our results suggest that water is going to be more plentiful in those regions of the world that are already `water-rich'. However, water stresses will increase significantly in regions and seasons that are already relatively dry. This could pose a very challenging problem for water-resource management around the world. For soil moisture, our results indicate reductions during much of the year in many semi-arid regions of the world, such as the southwestern region of North America, the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. In some of these regions, soil moisture values are reduced by almost a factor of two during the dry season. The drying in semi-arid regions is likely to induce the outward expansion of deserts to the surrounding regions. Over extensive regions of both the Eurasian and North American continents in high and middle latitudes, soil moisture decreases in summer but increases in winter, in contrast to the situation in semi-arid regions. For river discharge, our results indicate an average increase of ~ 15% during the next few centuries. The discharges from Arctic rivers such as the Mackenzie and Ob' increase by much larger fractions. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra also increase considerably. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller.
Delworth, Thomas L., Ronald J Stouffer, Keith W Dixon, Michael J Spelman, Thomas R Knutson, Anthony J Broccoli, P J Kushner, and Richard T Wetherald, 2002: Review of simulations of climate variability and change with the GFDL R30 coupled climate model. Climate Dynamics, 19(7), 555-574.
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A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near milennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.
Milly, P C., and Richard T Wetherald, 2002: Macroscale water fluxes 3. Effects of land processes on variability of monthly river discharge. Water Resources Research, 38(11), 1235, DOI:10.1029/2001WR000761.
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A salient characteristic of river discharge is its temporal variability. The time series of flow at a point on a river can be viewed as the superposition of a smooth seasonal cycle and an irregular, random variation. Viewing the random component in the spectral domain facilitates both its characterization and an interpretation of its major physical controls from a global perspective. The power spectral density functions of monthly flow anomalies of many large rivers worldwide are typified by a "red noise" process: the density is higher at low frequencies (e.g., <1y–1 ) than at high frequencies, indicating disproportionate (relative to uncorrelated "white noise") contribution of low frequencies to variability of monthly flow. For many high-latitude and arid-region rivers, however, the power is relatively evenly distributed across the frequency spectrum. The power spectrum of monthly flow can be interpreted as the product of the power spectrum of monthly basin total precipitation (which is typically white or slightly red) and several filters that have physical significance. The filters are associated with (1) the conversion of total precipitation (sum of rainfall and snowfall) to effective rainfall (liquid flux to the ground surface from above), (2) the conversion of effective rainfall to soil water excess (runoff), and (3) the conversion of soil water excess to river discharge. Inferences about the roles of each filter can be made through an analysis of observations, complemented by information from a global model of the ocean-atmosphere-land system. The first filter causes a snowmelt-related amplification of high-frequency variability in those basins that receive substantial snowfall. The second filter causes a relatively constant reduction in variability across all frequencies and can be predicted well by means of a semiempirical water balance relation. The third filter, associated with groundwater and surface water storage in the river basin, causes a strong reduction in high-frequency variability of many basins. The strength of this reduction can be quantified by an average residence time of water in storage, which is typically on the order of 20–50 days. The residence time is demonstrably influenced by freezing conditions in the basin, fractional cover of the basin by lakes, and runoff ratio (ratio of mean runoff to mean precipitation). Large lake areas enhance storage and can greatly increase total residence times (100 to several hundred days). Freezing conditions appear to cause bypassing of subsurface storage, thus reducing residence times (0–30 days). Small runoff ratios tend to be associated with arid regions, where the water table is deep, and consequently, most of the runoff is produced by processes that bypass the saturated zone, leading to relatively small residence times for such basins (0–40 days).
Milly, P C., Richard T Wetherald, Krista A Dunne, and Thomas L Delworth, 2002: Increasing risk of great floods in a changing climate. Nature, 415(6871), 514-517.
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Radiative effects of anthropogenic changes in atmospheric composition are expected to cause climate changes, in particular an intensification of the global water cycle with a consequent increase in flood risk. But the detection of anthropogenically forced changes in flooding is difficult because of the substantial natural variability; the dependence of streamflow trends on flow regime further complicates the issue. Here we investigate the changes in risk of great floods- that is, floods with discharges exceeding 100-year levels from basins larger than 200,000 km2- using both streamflow measurements and numerical simulations of the anthropogenic climate change associated with greenhouse gases and direct radiative effects of sulphate aerosols. We find that the frequency of great floods increased substantially during the twentieth century. The recent emergence of a statistically significant positive trend in risk of great floods is consistent with results from the climate model, and the model suggests that the trend will continue.
Soden, Brian J., Richard T Wetherald, Georgiy Stenchikov, and A Robock, 2002: Global cooling after the eruption of Mount Pinatubo: A test of climate feedback by water vapor. Science, 296(5568), 727-730.
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The sensitivity of Earth's climate to an external radiative forcing depends critically on the response of water vapor. We use the global cooling and drying of the atmosphere that was observed after the eruption of Mount Pinatubo to test model predictions of the climate feedback from water vapor. Here, we first highlight the success of the model in reproducing the observed drying after the volcanic eruption. Then, by comparing model simulations with and without water vapor feedback, we demonstrate the importance of the atmospheric drying in amplifying the temperature change and show that, without the strong positive feedback from water vapor, the model is unable to reproduce the observed cooling. These results provide quantitative evidence of the reliability of water vapor feedback in current climate models, which is crucial to their use for global warming projections.
Wetherald, Richard T., 2002: Greenhouse warming research In Encyclopedia of Physical Science and Technology, Vol. 7, 107-128.
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During the summer of 1988, one of the worst droughts in history occurred across most of the North American continent. During the subsequent winter, in the eastern United States, particularly in the mountainous watershed regions along the Appalachian range, very little snow fell. Since then, other anomalous weather events have occurred; severe flooding of the Mississippi River basin in the summer of 1993 and a series of abnormally dry summers and warm winters in the eastern United States with little or no snow in the late 1990s. Regardless of what caused these phenomena, they serve as graphic examples of what can happen if our climate changes significantly from that to which we have become accustomed. In particular, the summer of 1988, as well as an overall tendency for global warming since then, has sparked a great deal of discussion on the topic of greenhouse warming and whether or not it has actually begun. The Climate Dynamics Group of the Geophysical Fluid Dynamics Laboratory of NOAA, formerly headed by Dr. Syukuro Manabe, began researching the greenhouse effect in the late 1960s and early 1970s. During this period, data on atmospheric carbon dioxide (CO2 ) obtained by Dr. C. D. Keeling and his colleagues working at the Mauna Loa Observatory in Hawaii and Antarctica became available and indicated that atmospheric concentrations of CO2 were, indeed, increasing and increasing at a fairly consistent rate. These observations, coupled with the theoretical research work being done at the Geophysical Fluid Dynamics Laboratory (GFDL), laid the foundation for a transition of greenhouse theory from science fiction to science.
Wetherald, Richard T., and Syukuro Manabe, 2002: Simulation of hydrologic changes associated with global warming. Journal of Geophysical Research, 107(D19), 4379, DOI:10.1029/2001JD001195.
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Using the results obtained from a coupled ocean-atmosphere-land model with medium computational resolution, we investigated how the hydrology of the continents changes in response to the combined increases of greenhouse gases and sulfate aerosols in the atmosphere, which are determined based upon the IS92a scenario. In order to extract the forced response from natural, internal variability, the difference between the mean of an eight-member ensemble of numerical experiments and a control experiment are used for the present analysis. The global mean surface air temperature of the coupled model increases by about 2.3°C above the preindustrial level by the middle of the 21st century. Accompanying the warming, the global mean rates of both precipitation and evaporation increase by 5.2%, yielding the average increase in the rate of runoff by approximately 7.3%. The increase in the rate of runoff simulated by the model is particularly large in high northern latitudes, where the runoff from some rivers such as the Mackenzie and Ob´ may increase by as much as 20%. Runoff from many European rivers increases by more than 20%. Runoff also increases substantially in some tropical rivers such as the Amazon and Ganges. However, the percentage changes in simulated runoff from many other tropical rivers and middle latitude rivers are smaller with both positive and negative signs. In middle and high latitudes in the Northern Hemisphere, soil moisture tends to decrease in summer, whereas it increases in winter. However, in many semi-arid regions in subtropical and middle latitudes, soil moisture is reduced during most of a year. These semi-arid regions include the southwestern part of North America, the northeastern part of China in the Northern Hemisphere, and the region in the vicinity of the Kalahari Desert and southern part of Australia in the Southern Hemisphere. Since a semi-arid region usually surrounds a desert, the reduction of soil moisture in such a region often results in the expansion of the desert. Soil moisture is also reduced during the dry season in many semi-arid regions. For example, it is reduced in the savannahs of Africa and South America during winter and early spring in the Southern Hemisphere. In the Northern Hemisphere, it is reduced at the Mediterranean coast of Europe in summer.
Wetherald, Richard T., Ronald J Stouffer, and Keith W Dixon, 2001: Committed warming and its implications for climate change. Geophysical Research Letters, 28(8), 1535-1538.
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Time lags between changes in radiative forcing and the resulting simulated climate responses are investigated in a set of transient climate change experiments. Both surface air temperature (SAT) and soil moisture responses are examined. Results suggest that if the radiative forcing is held fixed at today's levels, the global mean SAT will rise an additional 1.0K before equilibrating. This unrealized warming commitment is larger than the 0.6K warming observed since 1990. The coupled atmosphere-ocean GCM's transient SAT response for the year 2000 is estimated to be similar to its equilibration response to 1980 radiative forcings - a lag of ~20 years. Both the time lag and the warming commitment are projected to increase in the future, and depend on the model's climate sensitivity, oceanic heat uptake, and the forcing scenario. These results imply that much of the warming due to current greenhouse gas levels is yet to be realized.
Delworth, Thomas L., Anthony J Broccoli, Keith W Dixon, Isaac M Held, Thomas R Knutson, P J Kushner, Michael J Spelman, Ronald J Stouffer, K Y Vinnikov, and Richard T Wetherald, 1999: Coupled climate modelling at GFDL: Recent accomplishments and future plans. Clivar Exchanges, 4(4), 15-20.
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Wetherald, Richard T., and Syukuro Manabe, 1999: Detectability of summer dryness caused by greenhouse warming. Climatic Change, 43(3), 495-511.
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This study investigates the temporal and spatial variation of soil moisture associated with global warming as simulated by long-term integrations of a coupled ocean-atmosphere model conducted earlier. Starting from year 1765, integrations of the coupled model for 300 years were performed for three scenarios: increasing greenhouse gases only, increasing sulfate-aerosol loading only and the combination of both radiative forcings. The integration with the combined radiative forcings reproduces approximately the observed increases of global mean surface air temperature during the 20th century. Analysis of this integration indicates that both summer dryness and winter wetness occur in middle-to-high latitudes of North America and southern Europe. These features were identified in earlier studies. However, in the southern part of North America where the percentage reduction of soil moisture during summer is quite large, soil moisture is decreased for nearly the entire annual cycle in response to greenhouse warming. A similar observation applies to other semi-arid regions in subtropical to middle latitudes such as central Asia and the area surrounding the Mediterranean Sea. On the other hand, annual mean runoff is greatly increased in high latitudes because of increased poleward transport of moisture in the warmer model atmosphere. An analysis of the central North American and southern European regions indicates that the time when the change of soil moisture exceeds one standard deviation about the control integration occurs considerably later than that of surface air temperature for a given experiment because the ratio of forced change to natural variability is much smaller for soil moisture compared with temperature. The corresponding lag time for runoff change is even greater than that of either precipitation or soil moisture for the same reason. Also, according to the above criterion, the inclusion of the effect of sulfate aerosols in the greenhouse warming experiment delays the noticeable change of soil moisture by several decades. It appears that observed surface air temperature is a better indicator of greenhouse warming than hydrologic quantities such as precipitation, runoff and soil moisture. Therefore, we are unlikely to notice definitive CO₂ -induced continental summer dryness until several decades into the 21st century.
Cess, R D., and Richard T Wetherald, et al., 1997: Comparison of the seasonal change in cloud-radiative forcing from atmospheric general circulation models and satellite observations. Journal of Geophysical Research, 102(D14), 16,593-16,603.
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We compare seasonal changes in cloud-radiative forcing (CRF) at the top of the atmosphere from 18 atmospheric general circulation models, and observations from the Earth Radiation Budget Experiment (ERBE). To enhance the CRF signal and suppress interannual variability, we consider only zonal mean quantities for which the extreme months (January and July), as well as the northern and southern hemispheres, have been differenced. Since seasonal variations of the shortwave component of CRF are caused by seasonal changes in both cloudiness and solar irradiance, the latter was removed. In the ERBE data, seasonal changes in CRF are driven primarily by changes in cloud amount. The same conclusion applies to the models. The shortwave component of seasonal CRF is a measure of changes in cloud amount at all altitudes, while the longwave component is more a measure of upper level clouds. Thus important insights into seasonal cloud amount variations of the models have been obtained by comparing both components, as generated by the models, with the satellite data. For example, in 10 of the 18 models the seasonal oscillations of zonal cloud patterns extend too far poleward by one latitudinal grid.
Haywood, J M., Ronald J Stouffer, Richard T Wetherald, Syukuro Manabe, and V Ramaswamy, 1997: Transient response of a coupled model to estimated changes in greenhouse gas and sulfate concentrations. Geophysical Research Letters, 24(11), 1335-1338.
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This study investigates changes in surface air temperature (SAT), hydrology and the thermohaline circulation due to the radiative forcing of anthropogenic greenhouse gases and the direct radiative forcing (DRF) of sulfate aerosols in the GFDL coupled ocean-atmosphere model. Three 300-year model integrations are performed with increasing greenhouse gas concentrations only, increasing sulfate aerosol concentrations only and increasing greenhouse gas and sulfate aerosol concentrations. A control integration is also performed keeping concentrations of sulfate and carbon dioxide fixed. The global annual mean SAT change when both greenhouse gases and sulfate aerosols are included is in better agreement with observations than when greenhouse gases alone are included. When the global annual mean SAT change from a model integration that includes only increases in greenhouse gases is added to that from a model integration that includes only increases in sulfate, the resulting global SAT change is approximately equal to that from a model integration that includes increases in both greenhouse gases and sulfate aerosol throughout the integration period. Similar results are found for global annual mean precipitation changes and for the geographical distribution of both SAT and precipitation changes indicating that the climate response is linearly additive for the two types of forcing considered here. Changes in the mid-continental summer dryness and thermohaline circulation are also briefly discussed.
Zhang, Y, Kikuro Miyakoda, and Richard T Wetherald, et al., November 1997: East Asian winter monsoon: results from eight AMIP models. Climate Dynamics, 13(11), 797-820.
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This study evaluates simulations of the East Asian winter monsoon in eight GCMs that participated in the Atmospheric Model Intercomparison Project (AMIP). In addition to validating the mean state of the winter monsoon, the cold surge and its transient properties, which includes the frequency, intensity, preferred propagation tracks, and the evolution patterns of the surges, are examined. GCM simulated temporal distribution of the Siberian high and cold surges is also discussed. Finally, the forcing of the cold surges on the tropical surface wind and convection, along with their interannual variation is analyzed. The mean state of the winter monsoon is generally portrayed well in most of the models. These include the climatological position of the Siberian high, the 200 hPa divergent center, and the large-scale wind patterns at the surface and the 200 hPa. Models display a wide range of skill in simulating the cold surge and its transient properties. In some of the models, the simulated cold surge trajectory, intensity, frequency, propagation patterns and source regions are in general agreement with those from the observed while in others, the models cannot adequately capture these observed characteristics. The temporal distribution of the Siberian high and cold surges were realistically reproduced in most GCMs. Most models were able to simulate the effect of the cold surges on the tropical surface wind, although a few models unrealistically generated subtropical southerly wind in the mid-winter. The relationship between cold surges and the tropical convection was not satisfactorily simulated in most models. The common discrepancies in the winter monsoon simulation can be attributed to many factors. In some models, the reason is directly related to the improper location of the large-scale convective center near the western Pacific. The satisfactory simulations of the monsoon circulation and the cold surges are partly due to the topographical characteristics of the East Asian continent, i.e., the Tibetan Plateau to the west and the oceans to the east. The correct simulation of the internnual variation of the surface wind near the South China Sea (SCS) and the maritime continent is a demanding task for most of the models. This will require adequate simulations of many aspects, including tropical convection, the Siberian cold dome, the extratropical-tropical linkage, and the air-sea interaction. The discrepancies noted here furnish a guide for the continuing improvement of the winter monsoon simulations. Improved simulations will lead to an adequate delineation of the surface wind and convection near the maritime continent, which is essential for portraying the winter monsoon forcing in a coupled model.
Zhang, Y, Kikuro Miyakoda, and Richard T Wetherald, et al., 1997: In GCM simulated East Asian winter monsoon: Results from eight AMIP models, PCMDI Report No. 39, Livermore, CA, PCMDI, 49 pp.
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This paper evaluates simulations of the East Asian winter monsoon in eight GCMs that participated in the Atmospheric Model Intercomparison Project (AMIP). In addition to validating the mean state of the winter monsoon, the cold surge and its transient properties, which includes the frequency, intensity, preferred propagation tracks, and the evolution patterns of the surges, are examined. GCM simulated temporal distribution of the Siberian high and cold surges is also discussed. Finally, the forcing of the cold surges on the tropical surface wind and convection, along with their interannual variation is analyzed. The mean state of the winter monsoon is generally portrayed well in the models considered. These include the climatological position of the Siberian high, the 200 hPa divergent center, and the large-scale wind patterns at the surface and 200 hPa. Models display a wide range of skill in simulating the cold surge and its transient properties. In some of the models, the simulated cold surge trajectory, intensity, frequency, propagaton patterns and source regions are in general agreement with those from the observed. While in others, the models cannot adequately capture these observed characteristics. The temporal distribution of the Siberian high and cold surges are realistically reproduced in most GCMs. Most models were able to simulate the effect of the cold surges on the tropical surface wind, although a few models unrealistically generated subtropical southerly wind in the mid-winter. The common discrepancies in the winter monsoon simulation can be atttributed to many factors. In some models, the inadequate resolution and the improper locations of the tropical convection are the most notable reasons. The satisfactory simulations of the monsoon circulation and the cold surges are partly due to the topographical characteristics of the East Asian continent, i.e., the Tibetan Plateau to the west and the oceans to the east. The correct simulation of the interannual variation of the surface wind near the South China 9Sea (SCS) and the maritime continent is a demanding task for most of the models. This will require adequate simulations of many aspects, including tropical convection, the Siberian cold dome, the extratropical-tropical linkage, and the air-sea interaction. The discrepancies noted here furnish a guide for the continuing improvement of the winter monsoon simulations. Improved simulations will lead to an adequate delineation of the surface wind and convection near the maritime continent, which is essential for portraying the winter monsoon forcing in a coupled model.
Cess, R D., and Richard T Wetherald, et al., 1996: Cloud feedback in atmospheric general circulation models: An update. Journal of Geophysical Research, 101(D8), 12,791-12,794.
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Six years ago, we compared the climate sensitivity of 19 atmospheric general circulation models and found a roughly threefold variation among the models; most of this variation was attributed to differences in the models' depictions of cloud feedback. In an update of this comparison, current models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the models still have physical disagreements.
Wetherald, Richard T., 1996: Feedback processes in the GFDL R30-14 level general circulation model In Climate Sensitivity to Radiative Perturbations: Physical Mechanisms and Their Validation, NATO ASI Series I, Vol. 34, Berlin, Heidelberg, Springer-Verlag, 251-266.
Yeh, T-C, Richard T Wetherald, and Syukuro Manabe, 1996: The effect of soil moisture on the short-term climate and hydrology change - A numerical experiment In Atmospheric Circulation to Global Change - Celebration of the 80th Birthday of Prof. YE Duzheng, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing: China, China Meteorological Press, 147-173.
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This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model with a limited computational domain and idealized geography. The soil at three latitude bands, namely 30° - 60° N, and 15° S - 15° N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, if the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist moisture anomaly for the 30° - 60° N case which is located under the middle latitude rainbelt tends to persist longer than the corresponding anomaly for the 0° - 30° N case. Although both the 30° - 60° N and 15° S - 15° N latitude regions occur under rainbelts, the soil moisture anomaly for the 15° S - 15° N case does not persist as long as it does for the 30° - 60° N case. This is because in the 15 degrees S - 15° N case, a much greater fraction of the increased precipitation is lost from the hydrologic cycle due to runoff there as compared with the 30° - 60° N case. The above changes of the hydrological processes also cause corresponding changes of the thermal state of the atmosphere such as a cooling of the surface due to increased evaporation. This results in changes of the mean zonal circulation through the thermal wind relationship. It is found that irrigation in the tropical region weakens the upward branch of the Hadley circulation in the vicinity of the tropical rainbelt.
Wetherald, Richard T., and Syukuro Manabe, 1995: The mechanisms of summer dryness induced by greenhouse warming. Journal of Climate, 8(12), 3096-3108.
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To improve understanding of the mechanisms]responsible for CO₂-induced, midcontinental summer dryness obtained by earlier modeling studies, several integrations were performed using a GCM with idealized geography. The simulated reduction of soil moisture in middle latitudes begins in late spring and is caused by the excess of evaporation over precipitation. The increase of carbon dioxide and the associated increase of atmospheric water vapor enhances the downward flux of terrestrial radiation at the continental surface at all latitudes. However, due mainly to the To improve understanding of the mechanisms responsible for CO₂-induced, midcontinental summer dryness obtained by earlier modeling studies, several integrations were performed using a GCM with idealized geography. The simulated reduction of soil moisture in middle latitudes begins in late spring and is caused by the excess of evaporation over precipitation. The increase of carbon dioxide and the associated increase of atmospheric water vapor enhances the downward flux of terrestrial radiation at the continental surface at all latitudes. However, due mainly to the CO₂-induced change in midtropospheric relative humidity, the increase in the downward flux of terrestrial radiation is larger in the equatorward side of the rain belt, making more energy available there for both sensible and latent heat. Since the saturation vapor pressure at the surface increases nonlinearly with surface temperature, a greater fraction of the additional radiative energy is realized as latent heat flux at the expense of sensible heat. Therefore, evaporation increases more than precipitation over the land surface in the equatorward side of the rain belt during spring and early summer and initiates the drying of the soil there. As the rain belt moves poleward from spring to summer, the soil moisture decreases in middle latitudes, reducing the rate of evaporation. This reduction of evaporation, in turn, causes a corresponding decrease of precipitation in middle latitudes, keeping the soil dry throughout the summer. In high latitudes, there is also a tendency for increased summer dryness. As noted in our previous studies, this feature mainly results from the earlier removal of highly reflective snow cover in spring, which enhances the evaporation in the late spring, lengthening the period of drying during the summer season. A similar mechanism also operates in middle latitudes, but its contribution is relatively small. The drying of soil is also enhanced by the land surface - cloud interaction in both middle and high latitudes. Owing to the reduction of cloud cover that results from the decrease of relative humidity in the lower troposphere, solar radiation absorbed by the continental surface increases, thereby enhancing evaporation and further reducing the soil moisture in summer. Although there is additional radiative energy available at the surface during winter, a greater fraction of it occurs as sensible heat rather than latent heat due to the colder surface temperature, thereby causing evaporation to increase less than precipitation. Because of the increased evaporation from the oceanic surface upstream whose temperature is warmer than the continental region in winter, precipitation over most of the continent increases substantially.
Wetherald, Richard T., and Brian J Soden, 1995: General simulation of atmospheric temperature and moisture in the GFDL AMIP Experiment In Proceedings of the First International AMIP Scientific Conference, WCRP-92, WMO/TD No. 732, Geneva, Switzerland, World Meteorological Organization, 97-100.
Colman, R, B McAveney, and Richard T Wetherald, 1994: Sensitivity of the Australian surface hydrology and energy budgets to a doubling of CO₂. Australian Meteorological Magazine, 43, 105-116.
Abstract
Changes in surface temperature, hydrology and energy budgets are examined over Australia for an equlibrium doubled CO₂ experiment using the Bureau of Meteorology Research Centre (BMRC) general circulation model (GCM). Changes in the surface hydrology budget are compared with those modelled using the Geophysical Fluid Dynamics Laboratory GCM. The continent is divided into northern and southern regions. These regions display soil moisture maxima in summer and winter respectively in both models, which essentially reflects the simulated seasonality of precipitation and evaporation. In the modelled doubled CO₂ climate, the BMRC GCM finds a strong increase in soil moisture in northern Australia in summer, due to a large increase in precipitation. Both models find a decrease in soil moisture in southern Australia in winter. This may be contrasted with a summer drying generally found at mid-latitudes in the northern hemisphere by GCMs. The BMRC model shows the crucial change in southern Australia to be a decrease in autumn precipitation. The findings highlight the danger of considering precipitation changes alone when assessing climate change impacts, as large increases in precipitation throughout most of the year do not result in increases in soil moisture. The soil moisture/cloud feedback mechanism proposed for northern continents appears to operate in Australia as well, although does not extend to high-level clouds. Changes in the surface heat budget involve a general increase in downwards long wave radiation except for southern Australia in autumn. The model response generally produces a close `pairing' of long and short wave radiation changes, caused primarily by clouds and resulting in net radiative changes at the surface close to zero. This forces a similar pairing of latent and sensible heat changes to occur also.
Karoly, D J., J A Cohen, Gerald A Meehl, J F B Mitchell, Abraham H Oort, Ronald J Stouffer, and Richard T Wetherald, 1994: An example of fingerprint detection of greenhouse climate change. Climate Dynamics, 10, 97-105.
Abstract
As an example of the technique of fingerprint detection of greenhouse climate change, a multi-variate signal or fingerprint of the enhanced greenhouse effect is defined using the zonal mean atmospheric temperature change as a function of height and latitude between equilibrium climate model simulations with control and doubled CO₂ concentrations. This signal is compared with observed atmospheric temperature variations over the period 1963 to 1988 from radiosonde-based global analyses. There is a significant increase of this greenhouse signal in the observational data over this period. These results must be treated with caution. Upper air data are available for a short period only, possibly too short to be able to resolve any real greenhouse climate change. The greenhouse fingerprint used in this study may not be unique to the enhanced greenhouse effect and may be due to other forcing mechanisms. However, it is shown that the patterns of atmospheric temperature change associated with uniform global increases of sea surface temperature, with El Niño-Southern Oscillation events and with decreases of stratospheric ozone concentrations individually are different from the greenhouse fingerprint used here.
Randall, David A., and Richard T Wetherald, et al., 1994: Analysis of snow feedbacks in 14 general circulation models. Journal of Geophysical Research, 99(D10), 20,757-20,771.
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Snow feedbacks produced by 14 atmospheric general circulation models have been analyzed through idealized numerical experiments. Included in the analysis is an investigation of the surface energy budgets of the models. Negative or or weak positive snow feedbacks occurred in some of the models, while others produced strong positive snow feedbacks. These feedbacks are due not only to melting snow, but also to increases in boundary temperature, changes in air temperature, changes in water vapor, and changes in cloudiness. As a result, the net response of each model is quite complex. We analyze in detail the responses of one model with a strong positive snow feedback and another with a weak negative snow feedback. Some of the models include a temperature dependence of the snow albedo, and this has significantly affected the results.
Cess, R D., and Richard T Wetherald, et al., 1993: Uncertainties in carbon dioxide radiative forcing in atmospheric general circulation models. Science, 262(5137), 1252-1255.
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Global warming, caused by an increase in the concentrations of greenhouse gases, is the direct result of greenhouse gas-induced radiative forcing. When a doubling of atmospheric carbon dioxide is considered, this forcing differed substantially among 15 atmospheric general circulation models. Although there are several potential causes, the largest contributor was the carbon dioxide radiation parameterizations of the models.
Randall, David A., R D Cess, and Richard T Wetherald, et al., 1992: Intercomparison and interpretation of surface energy fluxes in atmospheric general circulation models. Journal of Geophysical Research, 97(D4), 3711-3724.
Abstract
We have analyzed responses of the surface energy budgets and hydrologic cycles of 19 atmospheric general circulation models to an imposed, globally uniform sea surface temperature perturbation of 4 K. The responses of the simulated surface energy budgets are extremely diverse and are closely linked to the responses of the simulated hydrologic cycles. The response of the net surface energy flux is not controlled by cloud effects; instead, it is determined primarily by the response of the latent heat flux. The prescribed warming of the oceans leads to major increases in the atmospheric water vapor content and the rates of evaporation and precipitation. The increased water vapor amount drastically increases the downwelling infrared radiation at the Earth's surface, but the amount of the change varies dramatically from one model to another.
Boer, G J., and Richard T Wetherald, et al., 1991: An Intercomparison of the Climates Simulated by 14 Atmospheric General Circulation Models, CAS/JSC Working Group on Numerical Experimentation, WCRP-58, WMO/TD No. 425, Geneva, Switzerland: World Meteorological Organization, 37 pp.
Abstract
Climatological information from fourteen atmospheric general circulation models is presented and compared in order to assess the ability of a broad group of models to simulate current climate. The quantities considered are cross-sections of temperature, zonal wind and meridional streamfunction together with latitudinal and global distributions of mean sea-level pressure and precipitation rate. The nature of the deficiencies in the simulated climates that are common to all models and those which differ among models is investigated, general improvement in the ability of models to simulate certain aspects of the climate is shown, consideration is given to the effect of increasing resolution on simulated climate and approaches to the understanding and reduction of model deficiencies are discussed,
Cess, R D., and Richard T Wetherald, et al., 1991: Interpretation of snow-climate feedback as produced by 17 general circulation models. Science, 253(5022), 888-892.
Abstract
Snow feedback is expected to amplify global warming caused by increasing concentrations of atmospheric greenhouse gases. The conventional explanation is that a warmer Earth will have less snow cover, resulting in a darker planet that absorbs more solar radiation. An intercomparison of 17 general circulation models, for which perturbations of sea surface temperature were used as a surrogate climate change, suggests that this explanation is overly simplistic.
Wetherald, Richard T., 1991: Changes of temperature and hydrology caused by an increase of atmospheric carbon dioxide as predicted by general circulation models In Global Climate Change and Life on Earth, New York, NY, Chapman and Hall, 1-17.
Abstract
During the summer of 1988, one of the worst droughts in history occurred across most of the North American continent. During the subsequent winter, in the eastern United States, particularly in the mountainous watershed regions along the Appalachian range, very little snow fell. Regardless of what caused these phenomena, they serve as graphic examples of what can happen if our climate changes significantly from what we have become accustomed to. In particular, the summer of 1988 has sparked a great deal of discussion on the greenhouse effect and whether or not is is beginning. The Climate Dynamics Group of the Geophysical Fluid Dynamics Laboratory of NOAA, headed by Dr. Syukuro Manabe, began researching the greenhouse effect in the late 1960s and early 1970s. During this period, the data on atmospheric carbon dioxide (CO2) of Keeling et al. (1989) working at the Mauna Loa Observatory in Hawaii and Antarctica indicated that concentrations of CO2 were, indeed, increasing at a fairly consistent rate. The foundation for a transition of greenhouse theory from science fiction to science fact had been laid.
Wetherald, Richard T., V Ramaswamy, and Syukuro Manabe, 1991: A comparative study of the observations of high clouds and simulations by an atmospheric general circulation model. Climate Dynamics, 5, 135-143.
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The importance of clouds in the upper troposphere (cirrus) for the sensitivity of the Earth's climate e.g., requires that these clouds be modeled accurately in general circulation model (GCM) studies of the atmosphere. Bearing in mind the lack of unambiguous quantitative information on the geographical distribution and properties of high clouds, the simulated distribution of upper tropospheric clouds in a spectral GCM is compared with several satellite-derived datasets that pertain to high clouds only, for both winter and summer seasons. In the model, clouds are assumed to occupy an entire grid box whenever the relative humidity exceeds 99%: otherwise the grid box is assumed to be free of cloud. Despite the simplicity of the cloud prediction scheme, the geographical distribution of the maxima in the model's upper tropospheric cloud cover coincides approximately with the regions of the observed maxima in the high cloud amount and their frequency of occurrence (e.g., intertropical convergence zone and the monsoon areas). These areas exhibit a minimum in the outgoing longwave radiation (OLR; Nimbus-7) and are also coincident with regions of heavy precipitation. The model, with its relatively simple cloud formation scheme, appears to capture the principal large-scale features of the tropical convective processes that are evident in the satellite and precipitation datasets, wherein the intense, upward motion is accompanied by condensation and the spreading of thick upper tropospheric layers of high relative humidity and.cloudiness in the vicinity of the tropical rainbelt regions.
Wetherald, Richard T., and Syukuro Manabe, 1990: Hydrologic sensitivity to CO₂-induced global warming. Civil Engineering Practice Journal, 5(1), 33-36.
Abstract
The subject of possible hydrologic change in response to a CO₂-induced warming of the earth's atmosphere has received increasing attention at various scientific institutions in North America and Europe. Because changes of precipitation and evaporation could have a major impact on various aspects of our environment, scientific investigations have been concentrating upon the geographical details of CO₂-induced changes of hydrology. In order to better determine the effects of this warming, global circulation models provide a means of obtaining results that can be employed in the analysis of climatic changes.
Wetherald, Richard T., and Syukuro Manabe, 1988: Cloud feedback processes in a general circulation model. Journal of the Atmospheric Sciences, 45(8), 1397-1415.
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The influence of the cloud feedback process upon the sensitivity of climate is investigated by comparing the behavior of two versions of a climate model with predicted and prescribed cloud cover. The model used for this study is a general circulation model of the atmosphere coupled with a mixed layer model of the oceans. The sensitivity of each version of the model is inferred from the equilibrium response of the model to a doubling of the atmospheric concentration of carbon dioxide. It is found that the cloud feedback process in the present model enhances the sensitivity of the model climate. In response to the increase of atmospheric carbon dioxide, cloudiness increases around the tropopause and is reduced in the upper troposphere, thereby raising the height of the cloud layer in the upper troposphere. This rise of the high cloud layer implies a reduction of the temperature of the cloud top and, accordingly, of the upward terrestrial radiation from the top of the model atmosphere. Thus, the heat loss from the atmosphere-earth system of the model is reduced. As the high cloud layer rises, the vertical distribution of cloudiness changes, thereby affecting the absorption of solar radiation by the model atmosphere. At most latitudes the effect of reduced cloud amount in the upper troposphere overshadows that of increased cloudiness around the tropopause, thereby lowering the global mean planetary albedo and enhancing the CO₂ induced warming. On the other hand, the increase of low cloudiness in high latitudes raises the planetary albedo and thus decreases the CO₂ induced warming of climate. However, the contribution of this negative feedback process is much smaller than the effect of the positive feedback process involving the change of high cloud. The model used here does not take into consideration the possible change in the optical properties of clouds due to the change of their liquid water content. In view of the extreme idealization in the formulation of the cloud feedback process in the model, this study should be regarded as a study of the mechanisms involved in this process rather than the quantitative assessment of its influence on the sensitivity of climate.
Manabe, Syukuro, and Richard T Wetherald, 1987: Large-scale changes of soil wetness induced by an increase in atmospheric carbon dioxide. Journal of the Atmospheric Sciences, 44(8), 1211-1235.
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The change in soil wetness in response to an increase of atmospheric concentration of carbon dioxide is investigated by two versions of a climate model which consists of a general circulation model of the atmosphere and a static mixed layer ocean. In the first version of the model, the distribution of cloud cover is specified whereas it is computed in the second version incorporating the interaction among cloud cover, radiative transfer and the atmospheric circulation. The CO₂-induced changes of climate and hydrology are evaluated based upon a comparison between two quasi-equilibrium climates of a model with a normal and an above normal concentration of atmospheric carbon dioxide. It is shown that, in response to a doubling (or quadrupling) of atmospheric carbon dioxide, soil moisture is reduced in summer over extensive midcontinental regions of both North America and Eurasia in middle and high latitudes. Based upon the budget analysis of heat and water, the physical mechanisms responsible for the CO₂-induced changes of soil moisture are determined for the following four regions: northern Canada, northern Siberia, the Great Plains of North America and southern Europe. It is found that, over northern Canada and northern Siberia, the CO₂-induced reduction of soil moisture in summer results from the earlier occurrence of the snowmelt season followed by a period of intense evaporation. Over the Great Plains of North America, the earlier termination of the snowmelt season also contributes to the reduction of soil moisture during the summer season. In addition, the rainy period of late spring ends earlier, thus enhancing the CO₂-induced reduction of soil moisture in summer. In the model with variable cloud cover, the summer dryness over the Great Plains is enhanced further by a reduction of cloud amount and precipitation in the lower model atmosphere. This reduction of cloud amount increases the solar energy reaching the continental surface and the rate of potential evaporation. Both the decrease of precipitation and the increase of potential evaporation further reduce the soil moisture during early summer and help to maintain it at a low level throughout the summer. Over southern Europe, the CO₂-induced reduction of soil wetness occurs in a qualitatively similar manner, although the relative magnitude of the contribution from the change in snowmelt is smaller. During winter, soil moisture increases poleward of 30°N in response to an increase of atmospheric carbon dioxide. Because of the CO₂-induced warming, a greater fraction of the total precipitation occurs as rainfall rather than snowfall. The warmer atmosphere also causes the accumulated snow cover to melt during winter. Both processes act to increase the soil moisture in all four regions during the winter season. The increase of soil moisture is enhanced further in high latitudes due to the increase of precipitation resulting from the penetration of warm, moisture-rich air into higher latitudes. The CO₂-induced warming of the lower model troposphere increases with increasing latitude. The present analysis suggests that the changes of soil wetness described in this investigation are controlled by the latitudinal profile of the warming and are very broad scale, mid-continental phenomena.
Manabe, Syukuro, and Richard T Wetherald, 1986: Reduction in summer soil wetness induced by an increase in atmospheric carbon dioxide. Science, 232, 626-628.
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The geographical distribution of the change in soil wetness in response to an increase in atmospheric carbon dioxide was investigated by using a mathematical model of climate. Responding to the increase in carbon dioxide, soil moisture in the model would be reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, western Europe, northern Canada, and Siberia. These results were obtained from the model with predicted cloud cover and are qualitatively similar to the results from several numerical experiments conducted earlier with prescribed cloud cover.
Wetherald, Richard T., and Syukuro Manabe, 1986: An investigation of cloud cover change in response to thermal forcing. Climatic Change, 8, 5-23.
Abstract
The role of cloud cover in determining the sensitivity of climate has been a source of great uncertainty. This article reviews the distributions of cloud cover change from several climate sensitivity experiments conducted at the Geophysical Fluid Dynamics Laboratory of NOAA (GFDL) and other institutions. Two of the sensitivity experiments conducted at GFDL used a general circulation model with a limited computational domain and idealized geography, whereas three other experiments were conducted by the use of a global model with realistic geography. A thermal forcing imposed was either a change of solar constant or that of the CO₂-concentration in the atmosphere. It was found that in all five cases, clouds were decreased in the moist, convectively active regions such as the tropical and middle latitude rainbelts, whereas they increased in the stable region near the model surface from middle to higher latitudes. In addition, cloud also increased in the lower model stratosphere and generally decreased in the middle and upper troposphere for practically all latitudes. A comparison of the cloud changes obtained from investigations carried out at other institutions reveals certain qualitative (but not necessarily quantitative) similarities to the GFDL results. These similarities include a general reduction of tropospheric cloud cover especially in the vicinity of the rainbelts, a general increase of lower stratospheric cloud cover for almost all latitudes and an increase of low stratiform cloud in high latitudes.
Manabe, Syukuro, and Richard T Wetherald, 1985: CO₂ and hydrology. Advances in Geophysics, 28A, 131-157.
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Yeh, T-C, Richard T Wetherald, and Syukuro Manabe, 1984: The effect of soil moisture on a short-term climate and hydrology change--A numerical experiment. Monthly Weather Review, 112(3), 474-490.
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This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model with a limited computational domain and idealized geography. The soil at three latitude bands, namely 30 degrees N-60 degrees N, 0-30 degrees N, and 15 degrees S-15 degrees N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, if the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist moisture anomaly for the 30 degrees N-60 degrees N case which is located under the middle latitude rainbelt tends to persist longer than the corresponding anomaly for the 0-30 degrees N case. The soil at three latitude bands, namely 30 degrees N-60 degrees N, 0-30 degrees N, and 15 degrees S-15 degrees N is initially saturated with moisture. Although both the 30 degrees N-60 degrees N and 15 degrees S-15 degrees N latitude regions occur under rainbelts, the soil moisture anomaly for the 15 degrees S-15 degrees N case does not persist as long as it does for the 30 degrees N-60 degrees N case. This is because in the 15 degrees S-15 degrees N case, a much greater fraction of the increased precipitation is lost from the hydrologic cycle due to runoff there as compared with the 30 degrees N-60 degrees N case. The above changes of the hydrological processes also cause corresponding changes of the thermal state of the atmosphere such as a cooling of the surface due to increased evaporation. This results in changes of the mean zonal circulation through the thermal wind relationship. It is found that irrigation in the tropical region weakens the upward branch of the Hadley circulation in the vicinity of the tropical rainbelt.
Yeh, T-C, Richard T Wetherald, and Syukuro Manabe, 1983: A model study of the short-term climatic and hydrologic effects of sudden snow-cover removal. Monthly Weather Review, 111(5), 1013-1024.
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This paper describes the results from a set of numerical experiments which stimulate the effect of a large-scale removal of snow cover in middle and high latitudes during the early spring season. This is done through use of a simplified general circulation model with a limited computational domain and idealized geography. It is found that removal of snow cover reduces the water available to the soil through snowmelt and decreases soil moisture in this region during the following seasons. Furthermore, it also reduces surface albedo in this region and increases absorption of insolation by the ground surface. This, in turn, heats the ground surface and allows more evaporation to occur. However, the change of evaporation is relatively small owing to the low values of surface temperature in high latitudes. Therefore, the negative anomaly of soil moisture induced initially by the removal of snow cover persists for the entire spring and summer seasons. The removal of snow cover also affects the thermal and dynamical structure of the atmosphere. It is found that the increase of surface temperature extends into the upper troposphere thereby reducing both meridional temperature gradient and zonal wind in high latitudes.
Manabe, Syukuro, Richard T Wetherald, and Ronald J Stouffer, 1981: Summer dryness due to an increase of atmospheric CO₂ concentration. Climatic Change, 3, 347-386.
Abstract
To investigate the hydrologic changes of climate in response to an increase of CO₂-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat land surface. The second model has a global computational domain with realistic geography. The third model is identical to the second model except that it has a higher computational resolution. In each numerical experiment, the CO₂ -induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times the normal concentration of carbon dioxide in the air. It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and high latitudes in response to the increase of the CO₂ -concentration in air. This CO₂-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases.
Meleshko, V, and Richard T Wetherald, 1981: The effect of a geographical cloud distribution on climate: A numerical experiment with an atmospheric general circulation model. Journal of Geophysical Research, 86(C12), 11,995-12,014.
Abstract
This study is an attempt to stimate the effect of a geographical distribution of clouds on climate. A method of determination of a three-dimensional cloud distribution is proposed. It is based on the solution of the inverse problem for the radiative transfer equation. By using climatic data on total cloud amount, temperature, mixing ratio of water vapor, and satellite data on outgoing longwave radiation, the global distributions of high, middle, and low clouds were computed for July. The derived vertical cloud extension is in fair agreement with available data on the frequency distribution of stratiform and cumulus clouds. Two numerical experiments are carried out with an atmospheric general circulation model in which zonal and geographical cloud distributions are prescribed. The integrations are performed for 60 days with a GFDL model, and the last 30 days are analyzed. The geographical cloud distribution causes the increase of surface temperature over the continents by 2 degrees-4 degrees and leads to a decrease of surface pressure there and an increase over the oceans. The largest changes in the surface pressure, up to plus or minus 12 mbar, occur in the middle latitudes of both hemispheres. The largest differences in precipitation are observed in the tropics and over some coastal regions of North and South America. Arid areas in the subtropical belt become more pronounced in case of the geographical distribution of clouds. Estimates of the level of significance for precipitation and surface pressure changes reveal that they are statistically significant in some areas of the globe.
Wetherald, Richard T., and Syukuro Manabe, 1981: Influence of seasonal variation upon the sensitivity of a model climate. Journal of Geophysical Research, 86(C2), 1194-1204.
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To investigate the hydrologic changes of climate in response to an increase of CO₂-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat land surface. The second model has a global computational domain with realistic geography. The third model is identical to the second model except that it has a higher computational resolution. In each numerical experiment, the CO₂ -induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times the normal concentration of carbon dioxide in the air. It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and high latitudes in response to the increase of the CO₂ -concentration in air. This CO₂-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases.
Manabe, Syukuro, and Richard T Wetherald, 1980: On the distribution of climate change resulting from an increase in CO₂ content of the atmosphere. Journal of the Atmospheric Sciences, 37(1), 99-118.
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A study of the climatic effect of doubling or quadrupling of CO₂ in the atmosphere has been continued by the use of a simple general circulation model with a limited computational domain, highly idealized geography, no seasonal variation of insolation, and a simplified interaction between cloud and radiative transfer. The results from the numerical experiments reveal that the response of the model climate to an increase of CO₂ content in air is far from uniform geographically. For example, one can identify the high-latitude region of the continent where the runoff rate increases markedly, a zonal belt of decreasing soil moisture around 42 degrees latitude, and a zone of enhanced wetness along the east coast of the subtropical portion of the model continent. The general warming and the increase of moisture content of air, which results from a CO₂ increase, contributes to the large reduction of the meridional temperature gradient in the lower model troposphere because of 1) poleward retreat of highly reflective snow cover and 2) large increase in the poleward transport of latent heat. The reduction of the meridional temperature gradient appears to reduce not only the eddy kinetic energy, but also the variance of temperature in the lower model troposphere. The penetration of moisture into higher latitudes in the CO₂-rich warm climate is responsible for the large increase of the rates of precipitation and runoff in high latitudes of the model.
Wetherald, Richard T., and Syukuro Manabe, 1980: Cloud cover and climate sensitivity. Journal of the Atmospheric Sciences, 37(7), 1485-1510.
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This study discusses how the sensitivity of climate may be affected by the variation of cloud cover based on the results from numerical experiments with a highly simplified, three-dimensional model of the atmospheric general circulation. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized geography, no heat transport by ocean currents and no seasonal variation. Two versions of the model are constructed. The first version includes prognostic schemes of cloud cover and its radiative influences, and the second version uses a prescribed distribution of cloud cover for the computation of radiative transfer. Two sets of equilibrium climates are obtained from the long-term integrations of both versions of the model for several values of the solar constant. Based on the comparison between the variable and the fixed cloud experiments, the influences of cloud cover variation on the response of a model climate to an increase of the solar constant are identified. It is found that, in response to an increase of the solar constant, cloudiness diminishes in the upper and middle troposphere at most latitudes and increases near the earth's surface and the lower stratosphere of the model particularly in higher latitudes. Because of the changes described above, the total cloud amount diminishes in the region equatorward of 50 degrees latitude with the exception of a narrow sub-tropical belt. However, it increases in the region poleward of this latitude. In both regions, the area mean change in the net incoming solar radiation, which is attributable to the cloud-cover change described above, is approximately compensated by the corresponding change in the outgoing terrestrial radiation at the top of the model atmosphere. For example, equatorward of 50 degrees latitude, the reduction of both cloud amount and effective cloud-top height contributes to the increase in the area-mean flux of outgoing terrestrial radiation and compensates for the increase in the flux of net incoming solar radiation caused by the reduction of cloud amount. Poleward of 50 degrees latitude, the increase of cloudiness contributes to the reduction of both net incoming solar and outgoing terrestrial fluxes at the top of the model atmosphere. Although the effective cloud-top height does not change as it does in lower latitudes, the changes of these fluxes approximately compensate each other because of the smallness of insolation in high latitudes. Owing to the compensations mentioned above, the changes of cloud cover have a relatively minor effect on the sensitivity of the area-mean climate of the model.
Wetherald, Richard T., and Syukuro Manabe, 1979: Sensitivity studies of climate involving changes in CO₂ concentration In Man's Impact on Climate, New York, NY, Elsevier/North-Holland, Inc., 57-64.
Abstract
Attempts are made to estimate the temperature changes resulting from increasing the present CO₂ concentration by the use of: (a) a one-dimensional radiative convective equilibrium model and, (b) a simplified three-dimensional general circulation model. The following assumptions are made in the 3-D model: a limited computational domain, an idealized topography, zero surface heat capacity, no heat transport by ocean currents and an annual mean insolation. In general, the CO₂ increase raises the temperature of the model troposphere, whereas, it lowers that of the model stratosphere for both the 1-D and 3-D models. It is found that the tropospheric warming is somewhat larger for the 3-D model as compared with that obtained from the 1-D radiative convective equilibrium model. In particular, the increase of surface temperature in the 3-D model in high latitudes is magnified due to the recession of the snow boundary and the thermal stability of the lower troposphere which limits convective heating to the lowest layer. It is also found that increasing the CO₂ concentration significantly increases the overall intensity of the hydrologic cycle of the 3-D model. However, this does not necessarily imply the increase of wetness everywhere in the model region. In particular, the sign of wetness change depends upon the geographical location within the model domain.
Manabe, Syukuro, and Richard T Wetherald, 1975: The effects of doubling CO₂ concentration on the climate of a general circulation model. Journal of the Atmospheric Sciences, 32(1), 3-15.
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An attempt is made to estimate the temperature changes resulting from doubling the present CO₂ concentration by the use of a simplified three-dimensional general circulation model. This model contains the following simplifications: a limtied computational domain, an idealized topography, no heat transport by ocean currents, and fixed cloudiness. Despite these limitations, the results from this computation yield some indication of how the increase of CO₂ concentration may affect the distribution of temperature in the atmosphere. It is shown that the CO₂ increase raises the temperature of the model troposphere, whereas it lowers that of the model stratosphere. The tropospheric warming is somewhat larger than that expected from a radiative-convective equililbrium model. In particular, the increase of surface temperature in higher latitudes is magnified due to the recession of the snow boundary and the thermal stability of the lower troposphere which limits convective heating to the lowest layer. It is also shown that the doubling of carbon dioxide significantly increases the intensity of the hydrologic cycle of the model.
Wetherald, Richard T., and Syukuro Manabe, 1975: The effects of changing the solar constant on the climate of a general circulation model. Journal of the Atmospheric Sciences, 32(11), 2044-2059.
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A study is conducted to evaluate the response of a simplified three-dimensional model climate to changes of the solar constant. The model explicitly computes the heat transport by large-scale atmospheric disturbances. It contains the following simplifications: a limited computational domain, an idealized topography, no heat transport by ocean currents, no seasonal variation, and fixed cloudiness. It is found that the temperature of the model troposphere increases with increasing solar radiation. The greatest increase occurs in the surface layer of higher latitudes due to the effects of the snow-cover feedback mechanism as well as the suppression of vertical mixing by a stable lower troposphere. This result is found to be qualitatively similar to that obtained from previous studies with one-dimensional zonal mean models. One of the most interesting features of this investigation is the extreme sensitivity of the intensity of the computed hydrologic cycle to small changes of the solar constant. Current estimates indicate a 27% increase of the former as compared with a 6% increase of the latter. This large intensification of the hydrologic cycle in the model atmosphere results from the increase in the rate of evaporation which is caused by the following changes: 1) reduction of the Bowen ratio due to the nonlinear increase of saturation vapor pressure with increasing temperature at the earth's surface, and 2) decrease in the net upward terrestrial surface radiation resulting from the increase in the moisture content in air and from the reduction of the lapse rate (both of which increase the downward terrestrial radiation and increase the energy available for evaporation). It is shown that the latitude of maximum snowfall retreats poleward as the solar constant is increased. Furthermore, the total amounts of snowfall and snow accumulation decrease markedly with increasing insolation due to the poleward shift of the region of subfreezing surface temperature away from the zone of maximum baroclinic instability.
Wetherald, Richard T., and Syukuro Manabe, 1972: Response of the joint ocean-atmosphere model to the seasonal variation of the solar radiation. Monthly Weather Review, 100(1), 42-59.
Abstract PDF
The effect of the seasonal variation of solar radiation is incorporated into the joint ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration, and the resulting system is integrated for the 1 1/2-yr model time. The purpose of this study is to analyze the response of the joint air-sea model to seasonal changes in the solar zenith angle rather than to obtain a true equilibrium state. Comparisons are also made with results previously presented for the case of annual mean conditions. The most important feature that emerges as a direct result of this seasonal variation is a significant warming of the lower troposphere in high latitudes. This warming is found to be caused by (1) the removal of the snowpack during the summer season, which decreases the earth's albedo there during this time, and (2) a net rise in the temperature of the ocean surface in high latitudes as a result of the seasonal variation of convective activity in the surface layer of the ocean. The present results indicate that the snow cover effect is the primary factor responsible for this warming trend whereas the ocean effect is of secondary importance. The main consequences of this high latitude warming include a reduction of the mean atmospheric north-south temperature gradient (and, therefore, a reduction of baroclinic instability in middle latitudes), a reduction of the mean oceanic meridional circulation, and a reduction of the atmospheric and oceanic poleward heat energy transports.
Carroll, A B., and Richard T Wetherald, 1967: Application of parallel processing to numerical weather prediction. Journal of the Association for Computing Machinery, 14(3), 591-614.
Abstract
The purpose of this study is to illustrate the application of a parallel network processing computing system to an important class of problems in hydrodynamics. The computing system selected for this study is a prototype of the SOLOMON parallel processing system (cited as SOLOMON II) which was developed at the Westinghouse Defense and Space Center, Baltimore, Maryland. Emphasis is placed on the problem of numerical weather prediction mainly because of the large data storage and manipulation required, plus the extensive numerical analysis that is involved. The mathematical basis for numerical weather forecasting lies in the principles of conservation of mass, momentum, and energy. From these principles, research meteorologists have devised equations which express the physical laws governing the atmosphere. These basic equations are nonlinear partial differential equations and are well suited for solution by the SOLOMON II system. The computational method is to replace the region of interest by a grid system and replace the continuous equations by their finite difference approximations. In this representation, the programmer can take maximum advantage of mode control, neighboring connections, variable geometry, and simultaneous operation of a network of processing elements. Another topic discussed is the numerical solution of elliptic partial differential equations by the parallel processing network. It is felt by the authors that a parallel processing system of this type will offer a significant increase in computational speed over that of a sequentially organized computing system in the field of fluid dynamics as well as in other scientific fields.
Manabe, Syukuro, and Richard T Wetherald, 1967: Thermal equilibrium of the atmosphere with a given distribution of relative humidity. Journal of the Atmospheric Sciences, 24(3), 241-259.
Abstract PDF
Radiative convective equilibrium of the atmosphere with a given distribution of relative humidity is computed as the asymptotic state of an initial value problem. The results show that it takes almost twice as long to reach the state of radiative convective equilibrium for the atmosphere with a given distribution of relative humidity than for the atmosphere with a given distribution of absolute humidity. Also, the surface equilibrium temperature of the former is almost twice as sensitive to change of various factors such as solar constant, CO₂ content, O₃ content, cloudiness, than that of the latter, due to the adjustment of water vapor content to the temperature variation of the atmosphere. According to our estimate, a doubling of the CO₂ content in the atmosphere has the effect of raising the temperature of the atmosphere (whose relative humidity is fixed) by about 2C. Our model does not have the extreme sensitivity of atmospheric temperature changes of CO₂ content which was adduced by Möller.

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